1. Field of the Invention
The present invention relates to a demodulating apparatus which receives as an input signal a digital phase modulated wave sent from a transmitter side and demodulates this to obtain reproduced data.
Data transmission systems based on digital phase modulation are made much use of in fields such as radio communication and satellite communication. Among the currently used data transmission systems, there are the binary phase shift keying system (BPSK) and the quadrature phase shift keying system (QPSK). The demodulating apparatuses of these are comprised of a plurality of functional blocks. The present invention relates to the functional block for carrier recovery. This carrier recovery functional block had to establish phase synchronization rapidly between the carrier of the input signal and the recovered carrier.
2. Description of the Related Art
As will be explained later in detail referring to the drawings, when using a conventional demodulating apparatus, when there was an instantaneous disconnection of the input signal, it became impossible to achieve quick pull-in of synchronization for the next arriving input signal.
For example, assume a first ground station (A) suspends communication with a second ground station (B) so as to start communication with a third ground station (C). At this time, the first ground station (A) temporarily enters a non-input signal state. When this happens, the later-mentioned up/down counter in the first ground station (A) receives a sweep signal from the sweep/track signal output unit (mentioned later) and immediately enters an up-count (or down-count) operation in one direction. That is, it enters the sweep mode. In this case, if the input signal from the third ground station (C) next starting the communication is received directly after the above-mentioned input signals are disconnected, there is a possibility that the track mode can be returned to once again, but when the input signal is received a while after the input signal is disconnected, the sweep in the sweep mode is considerably advanced and the signal is out of the synchronization pull-in range of the track mode. This being the case, since the pull-in of synchronization is started under the sweep mode anew, at the maximum, one cycle of the sweep passes (that is, the up/down counter ends one cycle's worth of the count in one direction). During that time, communication between the first ground station (A) and the third ground station (C) cannot be started until synchronization is established.
In the final analysis, there is the problem that several seconds are taken at each ground station after the input signal from a certain other ground station is disconnected until the input signal from still another ground station can start to be normally received. The time of these several seconds itself is short, but a blank of several seconds occurring in the middle of for example communication for a television conference would prolong the conference and significantly reduce the quality of the communication service.